Calibrating radiometers

ABSTRACT

Radiometers detect radio wavelenth electromagnetic radiation and typically have an antenna ( 16 ), an amplifier ( 18 ) and a detector ( 20 ). All three of these components have response characteristics that ma be dependent on temperature, and in the case of systems using radiometer arrays dependent upon temperatures throughout the system. Different temperatures across a multi-channel antenna and differential channel temperature response can result in poor image quality from imaging radiometers. Resolution of a linear array of detector horns is limited by the size of the horns. An imaging radiometer ( 10 ) uses a focussing polariser ( 36 ), a quarter wave plate ( 32 ), a rotating inclined disk ( 28 ), and a detector feed array ( 16 ) to perform a conical scan, and compensates for variations in operating temperature of a radiometer using one or more of a variety of techniques including calibrating channels relative to each other, calibrating channels using reference temperatures in situo, and calibrating channels using temperature response predictions stored in the radiometer signal processor ( 22 ). Aspects of the invention also optimise image resolution, image quality and allow calibration.

[0001] This invention relates to radiometers, especially but notexclusively imaging radiometers, real time passive mm wave imagers, andquarter wave plate passive millimetre wave imagers.

[0002] Radiometer scanner arrangements have different knownconfigurations, and size of scanner is a consideration.

[0003] Radiometers detect radio wavelength electromagnetic radiation andtypically have an antenna, an amplifier and a detector. An imagingradiometer (or passive millimetre wave imager) will have in additionfocussing optics. All the components have response characteristics thatmay be dependent on temperature, and in the case of the antennadependent upon differences in temperature across the receiver array.Different temperatures across a multi-channel receiver array anddifferential channel temperature response can result in poor imagequality.

[0004] Due to the fact that the output signals of the detector of aradiometer are to some extent dependent upon the temperature of theradiometer, and differences in temperature over the radiometer, it isnecessary to calibrate the radiometer to compensate or modify the outputsignals from the detector to obtain more reliable images of a scene. Aradiometer will typically have a main laboratory or factory calibrationperiodically to re-set it (for example every 4 months or so). Thistypically involves pointing the antenna at a hot source of knowntemperature (e.g. room ambient temperature), and pointing it at a coldsource of known temperature (e.g. liquid nitrogen). Software in theradiometer can then perform a main base calibration, setting the gainand offset voltage to be applied to the output voltage of the detector.

[0005] However, the gain and offset of a channel of a radiometer varieswith the temperature of the radiometer at the time of using it, and thiseffect can alter the gain by as much as 30%. There are other factorswhich can alter the gain and offset, but temperature can be a majorvariable factor. Ambient temperature changes can cause significantproblems for imaging radiometers. Even more importantly for an imagingradiometer is the fact that different antenna channels (or the sameantenna channel at different times) can have different temperatures,which can cause distortion of the output voltage of the detector(s) andhence image distortion. Furthermore, the amplifier can have differenttemperatures at different times during the detection process and thiswill influence the output voltage of the detector.

[0006] It is known to alleviate the above problems in a number of ways.One way is to strive to maintain the temperature of thetemperature-sensitive components of the radiometer constant duringoperation of the radiometer so that only changes in temperature seen bythe detector due to incident radiation (i.e. the mm wave radiation beingdetected) cause variation in detector output voltage. This approachworks: it is possible to provide the radiometer in a controlledenvironmental chamber and to maintain the whole radiometer at acarefully controlled temperature, for example a stable elevatedtemperature (elevated in comparison with typical ambient temperatures).However, the device is then not very user-friendly, and the externalchamber heating and/or cooling mechanism, and their servo controls, arecomplex and expensive, and susceptible to mechanical failure or damage.The device also has extra weight and bulk.

[0007] Another approach is to compare the scene signals with a referencesignal and use that reference to re-calibrate the radiometerperiodically. The radiometer may be re-calibrated between each frame ofan imaging radiometer. The reference signal could be provided by achopper, but this typically halves the sensitivity of the device(because the detectors spend half of their time looking at a referencechopper). Alternatively it is known to inject an electronic noise signalfrom a resistor into the detector signal, and to use this noise as areference signal. However, the gain of the amplifier used in the noisesource may change with temperature, and so the reference signal may notbe as stable as is desirable.

[0008] Other problems experienced by radiometers are that for mm waveradiometers, their detector feed horns are a finite size and this limitshow closely they can be spaced in a linear array. Furthermore, it issometimes desired to have more information content/discrimination in thesignal between notional pixels in an observed scene.

[0009] There is a pixel interrogation time in a scanning radiometer: ifit is too long the data can be unreliable due to changes in the observedscene. It is therefore desirable to keep the pixel integration time ofthe radiometer detector short.

[0010] It is an aim of the invention to provide a new radiometer. Theradiometer may at least ameliorate at least one of the aforesaiddifficulties. It is an aim of one embodiment of the invention to providea radiometer with improved calibration features to compensate fortemperature fluctuation, and optionally to provide thecalibration/compensation with no or few moving parts, and without addingsignificantly to the mass or size of the device.

[0011] It is an aim of another embodiment of the invention to improveresolution and/or picture quality.

[0012] Quarter Wave Plate Scanner

[0013] According to first aspect the invention comprises a scanningimaging radiometer comprising scanner components, a detector, and acontrol processor adapted to control the operation of the scannercomponents; wherein the scanner components comprise a focussing elementadapted in use to focus radiation onto the detector, a detector field ofview director comprising a reflector plate rotatable about an axis ofrotation with the plane of the reflector plate being inclined relativeto the normal of the axis of rotation, and a quarter wave plate provideddisposed between the reflector plate and the focussing element; andwherein the detector has a detector feed provided disposed between thefocussing element and the reflector plate.

[0014] Preferably the detector feed is provided between the focussingelement and the quarter wave plate. Preferably the quarter wave platecomprises a meanderline device.

[0015] Preferably the focussing element also comprises a polariseradapted in use to transmit radiation of one polarisation (e.g. of afirst plane of polarisation) and to reflect radiation of the orthogonalpolarisation.

[0016] The detector preferably comprises an array of feed elements. Eachdetector preferably receives radiation from a particular part of theobserved scene. The detector field of view director preferably, in use,directs radiation from a particular part of the observed scene to adetector feed (or from respective different parts of the observed sceneto respective different individual detector feeds).

[0017] Preferably the reflector is disposed closer to a scene-observingaperture of the radiometer than the detector, quarter wave plate, andreflector plate. Preferably, in order of physical position relative toan image-capturing aperture, the focussing element is closest, followedby the detector, followed by the quarter wave plate, followed by thereflector plate.

[0018] Preferably the radiometer is a conical scanner (selects circlesor annuli in space in a scene being observed and focuses points orsections (regions/areas defined by the optics of the device) disposed ondifferent annuli onto respective individual detector elements of thedetector array, different points (micro-areas) on any one annulus beingfocussed onto the same detector element, but at different moments intime. The conical scanner directs points/micro areas from a singleannulus onto an individual detector element using the field of viewdirector and focussing optics.

[0019] The focusing element may comprise a polariser, possibly a linearpolariser, but it could be another form of polariser, for example acircular polarisation device. The focussing element may focus radiationwith a first linear polarisation and not radiation with a polarisationdisplaced by 90°. The focusing element may comprise a dish of generallyparallel wires. The focussing element may be disposed in front of thedetector feed in relation to a scene capturing aperture of theradiometer. A polarisation changing element may be provided between thereflector and the focussing element, and may comprise a meanderlinedevice.

[0020] Offset Array

[0021] According to another aspect the invention comprises a scanningradiometer having a detector array, a scanner, and a control processoradapted to receive signals from the detector array, in which thedetector array has an elongate length, or curve, and detector array feedelements each feeding respective channels, said feed elements beingspaced along the elongate length, or curve, of the array, and in whichthe scanner is in use controlled by the control processor to scanregions of an observed scene over the detector array, and in which thedetector array comprises a first line or curve of feed elements withtheir centres spaced apart by a first distance and a second line, orcurve, of feed elements with the centres of feed elements of the secondline, or curve, spaced apart by a second distance, the centres of thefirst and second lines, or curves, of detector elements being offsetfrom each other in the elongate direction of the first line, or curve.

[0022] The lines of feed elements need not be straight: they could becurved.

[0023] The curves of curved detector arrays can be adjacent straightlinear arrays or adjacent annular arrays. In the case of linear arraysthe scan pattern in the imager will be linear displaced circles, and inthe case of annular arrays the scan pattern will be annular displacedcircles. The advantage of the annular array is that more samples aretaken in the centre of the image. The advantages of the linear array isthat it is easier to make.

[0024] The feed elements of a line, or curve, will usually beequidistantly spaced from each other, but might not be. The spacing ofthe feed elements of the first line, or curve, will usually be the sameas the spacing of the feed elements along the second line, or curve, butmight not be.

[0025] Preferably the second line, or curve, extends generally parallelto the first line. Preferably the first and second lines, or curves, ofdetector feeds are adjacent to each other, most preferably substantiallyas close to each other as the geometry of the feed elements permits.Preferably the first distance is substantially as small as the geometryof the feed elements permits. Preferably the first and second feedelements have substantially the same size. Preferably the first andsecond lines, or curves, comprise lines of substantially identical feedelements spaced substantially as close as possible to adjacent feedelements in their own line, or curve, and with the two lines, or curves,substantially as close as possible to each other, with an offset in therelative positions of the first and second lines, or curves, in theirelongate direction.

[0026] Having the detector elements as close as possible improves theresolution of the device.

[0027] The result of offsetting the detector elements in the second linerelative to the detector elements in the first line with the two linesbeing in nearly the same place in space, is effectively to produce amulti-line array which is equivalent to a single line array, but withcloser feed element spacing in the elongate direction of the array.

[0028] There is a limit to how close the centres of two feed elementscan be. They are typically rectangular horns and have a finite size.This can put a constraint on how close together the horns can be in aline, which in turn constrains the resolution of the device.

[0029] Offsetting the horns/fields of adjacent lines of horns/feedseffectively reduces the line spacing between the centre of two horns(the distance in the elongate direction between the horn centres).

[0030] The offset may be ½ pitch offset. This effectively halves theinter-feed spacing in the elongate direction. Alternatively a differentoffset may be used (e.g. ¼ or ⅓ pitch).

[0031] There may be only two lines of feed elements, but it is notnecessary to have only two lines of feed elements; more than two linescan be envisaged e.g. 3, 4 etc. If there are n similar lines of feedelements then the offset, the feed spacing, in the linear directionbetween the centres of feed elements in the lines may be 1/n of thepitch.

[0032] The direction of scanning of the scene over the detector arraymay be substantially perpendicular to the elongate direction of thedetector element lines. In non straight arrays the elongate directionmay be a local elongate direction, local to a few, or possibly only two,feed elements of the array. For example for any two adjacent feedelements in a line (curved) of feed elements there may be a line betweenthem, and an offset feed element from a different line may be disposedin that gap, when viewed projected along the line of radiation incidentupon the two adjacent feed elements.

[0033] The offset between centres of feed elements of different lines offeed elements may be such as to reduce the effective inter-feed elementcentre spacing in the elongate direction to Nyquist spacing, or nearNyquist spacing, or better than Nyquist spacing.

[0034] The Nyquist spacing does of course depend upon the frequency ofradiation being used. It is envisaged that the centre of the feedelements of a single line might be about λFn apart, and that the Nyquistspacing for radiation of interest might be $\frac{\lambda}{2}$

[0035] Fn, which means that two lines of feed elements at ½ pitchspacing should produce a system with about Nyquist spacing (where λ isthe free space wavelength of the radiation and Fn is the F number of thefocussing optics, and$\left. {{Fn} = \frac{{focal}\quad {length}}{{diameter}\quad {of}\quad {optics}}} \right).$

[0036] The radiometer is preferably an imaging radiometer.

[0037] According to another aspect the invention comprises a method ofimproving the resolution of an imaging radiometer of the kind having alinear array of detector feed elements and a scanner which scans anobserved scene over the linear array in a direction transverse to thelinear direction of the array, the method comprising providing an arrayhaving a plurality of lines of detector elements, with the lines beingoffset in the linear direction of the lines so that as seen along thelinear direction of the array as a whole the spacing of detector feedsis smaller than that found in a single line of feed elements, therebyeffectively reducing the spacing of detector feed elements in the lineardirection of the detector array.

[0038] Calibration

[0039] According to another aspect the invention comprises a radiometerhaving at least one detection channel and a detector adapted to receiveradiation acquired in use by the channel, a control processor, and anabsolute reference temperature provider, the arrangement being such thatthe radiometer can be put, in use, in a calibration mode in which itresets the gain and/or offset associated with the, at least one, oreach, channel by using signals derived from the reference temperatureprovider.

[0040] Preferably the absolute reference temperature provider comprisesa holder for holding a substance of known temperature.

[0041] There may be two (or more) different absolute referencetemperature providers, adapted to present different known temperaturesto the radiometer.

[0042] The reference temperature provider may include a temperaturesensor adapted to provide signals indicative of the temperature of thereference temperature provider to the control processor. The referencetemperature provider(s) may comprise a thermo-electric device.

[0043] The radiometer may have an array of detector feed elements andthe or each reference temperature provider may be provided adjacent tothe detector feed array. Preferably the detector feed array and itsread-out connections extend away from the detector elements and aredisposed in part of the optical path of radiation that enters theradiometer's image-acquiring aperture, and preferably the referencetemperature provider(s) are disposed in the same projected position asthe read out connections. This may minimise furtherdisruption/degradation of the scene radiation captured by the radiationsince the reference temperature provider(s) obscure only the same path(or substantially the same path) as the read out wiring.

[0044] Preferably the radiometer has a switch adapted to cause thedetector array to detect radiation from the reference temperatureprovider instead of from the scene. The switch may comprise apolarisation altering component adapted to have a scene-observingposition and a reference-temperature observing position. The switch maybe moved angularly between the reference temperature and scene observingpositions. The switch may comprise a quarter wave plate. The switch maycomprise a meanderline device. The meanderline device or ¼ plate mayhave a fast or slow axis aligned with the plane of linearly polarisedradiation during reference temperature measurement and aligned at 45°disposed to this when measuring scene radiation.

[0045] The radiometer may have a meanderline plate which for normalimaging of a scene is downstream of a linear polariser and has its fastor slow axis inclined at 45° to the plane of polarisation of radiationfrom the scene that has been linearly polarised prior to interactingwith the meanderline plate, and which when in reference temperatureobserving mode has its fast or slow axis generally parallel to the planeof polarisation of the radiation incident upon it.

[0046] The polarising reflector may be rotated by 90° on its axis toswitch the passive millimetre wave imaging from its normal imaging toone where all radiation channels view essentially unfocussed radiationfrom the scene which has effectively the same radiation temperature.

[0047] Preferably a reflector, possibly inclined relative to an axis ofrotation of the reflector, is provided downstream of the meanderlinedevice. Preferably a linear polariser is provided upstream of themeanderline device. The linear polariser may also focus radiation ontothe detector array.

[0048] Modifying Gain/Offset

[0049] According to another aspect the invention comprises a method ofimproving the image quality of a multi-channel imaging radiometer havingat least a first channel which detects radiation using a first detectorand a second channel which detects radiation using a second detector,the method comprising modifying the gain and/or offset used in thescene-temperature vs detector voltage equations for the first and secondchannels using values for the gain and offset derived by performing achannel calibration operation, and in that channel calibration operationensuring that the first and second channels observe substantially thesame temperature and using the outputs from the first and seconddetectors in the calibration operation to produce a modified value forthe gain and/or offset for the first and/or second channel; theradiometer using the modified gain and/or offset for the first and/orsecond channel to create an image using the first and second channels.

[0050] Preferably the first and second channels observe substantiallythe same scene temperature. Of course, the first and second channels maybe physically next to each other in a detector array, but they need notbe: they could be spaced apart physically and there could be otherchannels in between them. “First” and “second” channels could be read as“a channel” and “another different channel”.

[0051] Thus the factory/laboratory set gain and offset voltage for thefirst channel, and for the second channel, are re-evaluated in arelative channel calibration operation. It will be appreciated that thegain and offset for one channel could be left alone, as a reference,base level, and the gain and offset of the second channel set relativeto that. Alternatively, the gain and offset of both channels could bemodified/reset.

[0052] Preferably a third channel, or further channels, has its or theirgain and offset calibrated relative to the first or second channel byuse of a channel calibration operation in which two channels observesubstantially the same temperature and the detector signals from themare used to calibrate the gain and offset of one channel relative to theother.

[0053] Usually, both the gain and offset of a channel will be modifiedin the relative calibration operation, but sometimes it may be necessaryto modify only one of them, depending upon what the equations producesas the relative gains and offsets.

[0054] The two channels may conveniently observe the same temperature inthe calibration operation by observing substantially the same point inspace in the scene being observed at substantially the same time.Usually, there will be a slight time difference between first and secondchannel observations of the same point, but the temperature of thatpoint in the scene can be taken to be constant if the time difference issmall enough.

[0055] Preferably, the calibration operation comprises observing withthe first and second channels a first temperature, the same temperaturefor each channel, and a second temperature, the same temperature foreach channel, the second temperature being different from the firsttemperature. By having two different observed temperatures with the twochannels, with the first and second channels each having a respectivedetector signal response that is dependent upon gain and offset, it ispossible to solve the response equations for the relative gain andrelative offset.

[0056] The temperature of the observed temperature may be known, forexample because it is measured in some way, or it may be evaluated bythe radiometer, for example using the factory setting for a channel'sgain and offset. This observed temperature value may be used in theprocess of determining the relative gains and offsets of the channels.The observed temperature value used may be an average or a weightedfunction of the observed value temperature of the individual channelsthat are observing the same temperature.

[0057] The relative calibration operation may use the equations:

[0058] (i) Voltage of Detector 1=gain of channel 1×observed Temperature(channel 1)+Voltage offset for channel 1 and

[0059] (ii) Voltage of Detector 2=gain of channel 2×observed temperature(channel 2)+Voltage offset for channel 2 to determine one or more, orall, of: gain of channel 1, gain of channel 2, offset of channel 1, andoffset of channel 2.

[0060] Preferably equations (i) and (ii) are repeated for two differenttemperatures, observed temperature 1 and observed temperature 2, whichgives four equations:

V _(Det Channel 1 point A) =g ₁(t)×T _(observed A) +V ₀₁(t)  (equationa)

V _(Det Channel 2 point A) =g ₂(t)×T _(observed A) +V ₀₂(t)  (equationb)

V _(Det Channel 1 point B) =g ₁(t)×T _(observed B) +V ₀₁(t)  (equationc)

V _(Det Channel 2 point B) =g ₂(t)×T _(observed B) +V ₀₂(t)  (equationd)

[0061] and the method comprises solving the equations to determine thefour unknown variables g₁, V₀₁, g₂, V₀₂ (with T_(observed A) andT_(observed B), and the detector voltages known).

[0062] Preferably the relative calibration of the first and secondchannels takes place periodically in use of the radiometer, preferablyautomatically periodically, but a user may be able to initiatecalibration manually (e.g. by pressing a button), or may be able to setthe frequency of calibration. The radiometer may perform the relativechannel calibration at intervals of the order of:

[0063] (i) 50 ms or 100 ms; or

[0064] (ii) 1 second or a few seconds; or

[0065] (iii) 10 seconds or tens of seconds; or

[0066] (iv) a minute or few minutes; or

[0067] (v) ten minutes or more; or

[0068] (vi) between the construction of successive images; or

[0069] (vii) between every 1, 2, 3, 4, 5, 10, 20, 30, 40, 50, or more,images.

[0070] The method may comprise scanning a first track or path of anobserved scene onto the first channel and scanning a second track orpath of an observed scene onto the second channel, and intersecting thepaths. At the point of intersection the two channels are looking at acommon point in the scene.

[0071] Preferably the two tracks intersect at two, or two or more spacedapart points in the observed scene. It can be assumed that the twospaced apart points will be at different temperatures. This will be truefor many observed scenes.

[0072] Preferably the method comprising checking the pre-modificationvalues for the temperature being observed by the first and secondchannels to ensure that they are close enough to each other to beconsidered to be two channels measuring the same temperature.

[0073] If the two pre-modification values of the observed scenetemperature are outside of an allowable margin for their difference theradiometer may not re-calibrate those channels that time. If such anevent happens the radiometer may re-calibrate at a shorter interval, ifall channels are assessed for relative calibration are relativelycalibrated.

[0074] The temperature of the scene evaluated by each of the twochannels may be evaluated by a processor to determine whether theobserved scene temperature had varied between the time that the firstchannel observed it and the second channel observed it. Temperatureevaluated from a channel may be compared with the previous scene T forthat channel, or the scene T if evaluated using the other channel signal(or its own previous signal) to be used. If the processor decides that atemperature signal is suspect it may decide not to relatively calibratethe two channels on that decision.

[0075] The method may comprise making the two channels see the sametemperature by defocusing. Focusing or imaging optics (by “optics” norestriction to optical wavelength is intended) may be defocused so as toensure that the temperature seen by two channels is generally the same.

[0076] The method may comprise making the two channels see the sametemperature by switching the radiation incident upon the detectors frombeing scene radiation to reference radiation. The reference radiationmay be from a source associated with the radiometer. The referenceradiation may be provided by providing a thermal source to be viewed bythe channels. The thermal source may be provided at the imaging plane ofthe radiometer, possibly adjacent an imaging focal plane receiver feedarray. The thermal source may be provided by a thermoelectric device toprovide a thermal source of desired temperature. The temperature of thethermal source may be measured, and the temperature may be used todetermine the gain and offset for the channels.

[0077] According to another aspect the invention comprises a method ofcompensating a multi-channel radiometer for variations in the outputvoltage of a detector with temperature of the radiometer, comprisingmodifying the gain and offset voltage used in the evaluation of anobserved scene temperature by calibrating one channel's gain and offsetrelative to another channel's gain and offset.

[0078] Preferably the method comprises ensuring that the two channelsobserve the same temperature during a calibration operation. This may beachieved by having the channels observe the same point in the scene, orby having the channels observe a reference thermal source, or bydefocusing the scene.

[0079] According to another aspect the invention comprises amulti-channel imaging radiometer having a first channel having a firstchannel detector and being adapted to be connected to a first channelobserved scene radiation feed, and a second channel having a secondchannel detector and being adapted to be connected to a second channelobserved scene radiation feed, and a signal processor, in which observedscene radiation signals are adapted to be fed to the first and seconddetectors respectively by their first and second channels, and thedetectors are adapted in use to provide first and second detectoroutputs to the signal processor, the signal processor being adapted inuse to provide output signals representative of the temperaturesobserved in the observed scene by the first and second channels; and inwhich the signal processor is adapted in use to operate on the receivedfirst and second detector outputs using values representative of,influenced by, or associated with the gain and voltage offset for thefirst channel and for the second channel to calibrate the gain andvoltage offset for the second channel relative to those of the firstchannel.

[0080] Preferably the signal processor is adapted to use the equation:

V _(detector)=gain×observed scene temperature+V _(offset),

[0081] or an equivalent, for each channel to determine the modified gainand offset.

[0082] The first and second channel detectors are preferably differentdetectors, but they may be one and the same detector.

[0083] Preferably the radiometer is adapted to scan an observed scene insuch a way as to overlap or cross scan paths, so that the same point inthe observed scene is viewed by more than one channel. The radiometermay be adapted to perform a conical scan pattern.

[0084] If the scene temperature changes between the relative calibrationof two channels, or if it is predicted that it may change, then relativecalibration may be suspended until such time as it can be taken as beingconstant. For example, if the imager is panning or tilting the point inspace being observed by the channels will change between relativecalibration measurements, and so the temperature can be predicted not tobe constant. The radiometer may be arranged such that relativecalibration between channels does not occur when the observed source ismoved past the detector channels.

[0085] When a relative calibration is performed the gain and offsetevaluated may be examined or checked by a processor or software to makesure that the gains and offsets evaluated for the channels are notwildly different from (a) each other and/or (b) their previous values.Allowable limits for differences may be used to screen evaluated gainand offset. It is, of course, not necessary to calibrate every frame ofany image: one every few minutes may be enough.

[0086] The radiometer may be provided with an image-forming focuserassembly. There may be a defocuser adapted to defocus the detectedimage. The radiometer may have an observed radiation diverter adapted inuse to divert the radiation which encounters the detectors from beingradiation which has originated from the scene to being radiation that isnot from the observed scene. The alternative radiation may be providedby a thermal source provided with the radiometer or separate from it.The radiometer may have thermal source holding or providing means. Thethermal source may comprise a thermoelectric device, such as a Peltierdevice. There may be a temperature sensor adapted to measure thetemperature of the thermal source and to provide indicative signals tothe signal processor. A controller may control the current to athermoelectric reference temperature device to achieve a desiredreference temperature. The controller and the signal processor may bepart of the same processor.

[0087] According to another aspect the invention comprises a method ofimproving the accuracy of an image or of output signals image producedby a radiometer comprising periodically performing an absolutecalibration of the gain and offset voltage applicable to at least oneradiation detection channel against a source of known temperature, theabsolute calibration being performed with the radiometer in situo,without returning the device to the factory or the laboratory forcalibration.

[0088] Thus, the absolute calibration can be performed by the user,conveniently, without the interruption and expense of returning theradiometer to a remote site for calibration.

[0089] The source may be a thermal source, which may comprise the skyand the method may comprise ensuring that radiation from the sky isreceived by the detection channel. The thermal source may compriseliquid nitrogen. The thermal source may comprise a thermoelectricdevice.

[0090] Preferably, the method comprises observing two different thermalsource temperatures.

[0091] The method may comprise having a selector which selectivelydirects radiation from an observed scene onto the detector or radiationfrom the source of known temperature onto the detector.

[0092] According to another aspect the invention comprises a radiometerhaving at least one detection channel and a detector adapted to receiveradiation acquired in use by the channel, a control processor, and anabsolute reference temperature provider, the arrangement being such thatthe radiometer can be put, in use, in a calibration mode in which itresets the gain and/or offset associated with the, at least one, oreach, channel by using signals derived from the reference temperatureprovider.

[0093] Preferably the radiometer has a selector adapted in use to selectwhether radiation reaches the detector from the observed scene or fromthe reference temperature provider, which may be a thermal source ofknown temperature. There may be a defocuser. There may be a quarterwaveplate which can have its fast axis rotated by 45°.

[0094] According to another aspect the invention comprises a method ofimproving the performance of a radiometer having at least one detectionchannel comprising providing compensation for variations in temperatureof the radiometer, or at least for variation in temperature of at leastone component of the radiometer.

[0095] Preferably the method comprises providing an equivalence functionwhich in the evaluation of an observed scene temperature observed by thechannel compensates for the temperature of the radiometer, or said atleast one component. The component may be (i) an amplifier whichamplifies signals detected by a radiation-feed, or it may be (ii) adetector which detects signals, usually amplified signals, obtained froma radiation-feed, or it may be (iii) a radiation-feed, e.g. a horn orhorn array.

[0096] The method may comprise having a concordance between detectoroutput and evaluated scene temperature dependent upon the temperature ofthe radiometer or said component. The concordance may be a look-up tableor an algorithm.

[0097] The method may comprise having a concordance between the inputand output of any of (i), (ii), or (iii) dependent upon the temperatureof the component (i), (ii), or (iii).

[0098] The method may comprise using a modified output of the component(iii) as the input to component (i), and/or modified output of component(iii) as the input for component (ii), the outputs being modified inaccordance with the temperature-dependent performance of the components.

[0099] Alternatively or additionally, the evaluation of the temperatureof a point in an observed scene may comprise taking the output ofcomponent (ii) and applying a modification which uses one of more of:

[0100] (a) the input-output characteristics of component (ii) withtemperature; and/or

[0101] (b) the input-output characteristics of component (i) withtemperature; and/or

[0102] (c) the input-output characteristics of component (iii) withtemperature.

[0103] The input-output characteristics are preferably the gain andoffset attributable to the components in the equation

V _(detector)=gain×T _(scene) +V _(offset).

[0104] It is believed that the amplifier is the mosttemperature-sensitive component and the radiometer may compensate forchanges in temperature of this and not the horn or detector. It isbelieved that the detector is the next most temperature-sensitivecomponent and the radiometer may compensate for changes in temperatureof that, or that and the amplifier, and not for changes in temperatureof the horn.

[0105] The temperature dependency of the components could be predicted,but this is likely to be less desirable than measuring them. The methodmay comprise evaluating the temperature responsivity of the radiometer,or said at least one component, at one or more than one temperature inthe operational range of temperatures for the radiometer.

[0106] In the above way, a free-standing temperature correction is made:once the machine has been calibrated/its processor set up with theappropriate temperature-dependent corrections, they are applied to themeasured response to provide a modified response.

[0107] According to another aspect the invention comprises a radiometerhaving at least one detection channel having a gain and offset that aredependent upon the temperature of the radiometer and/or upon thetemperature of at least one component of the radiometer, a signalprocessor, and a temperature sensor adapted to detect the temperature ofthe radiometer or of said component; the channel being adapted in use toprovide signals indicative of the temperature in an observed scene tothe signal processor, and the signal processor being adapted in use toproduce evaluated scene temperature signals which are dependent uponboth the channel signal and upon the temperature sensor signal.

[0108] The channel may have an amplifier and a temperature sensor maysense the temperature of the amplifier. The channel will have a detectorand a temperature sensor may detect the temperature of the detector. Thechannel may have radiation-acquiring antenna, e.g. horn, and atemperature sensor may detect the temperature of that. The radiometermay have at least two or at least three temperature sensors providingsignals to the signal processor indicative of the temperature ofdifferent components.

[0109] The signal processor may have a concordance look-up table oralgorithm to produce the evaluated scene temperature signal, theconcordance linking output detector voltage with evaluated scenetemperature signal for different temperatures of the radiometer or ofthe or each component which has its temperature input to the signalprocessor.

[0110] According to another aspect the invention comprises a method ofcalibrating a radiometer comprising the steps of:

[0111] (i) providing a radiometer comprising a plurality of channels;

[0112] (ii) detecting a sample scan pattern for each of the channels;

[0113] (iii) overlapping at least two scan patterns;

[0114] (iv) sampling outputs of the channels at crossover points of thescan patterns;

[0115] (v) referencing gain and offset parameters at the overlap of thescan patterns.

[0116] The output of one of the channels may be compared with apredetermined reference value in order to obtain gain and offsetparameters.

[0117] According to another aspect the invention comprises a method ofcalibrating a radiometer comprising the steps of:

[0118] (i) storing parameter values for a radiometer at a range of knowntemperatures;

[0119] (ii) determining the functional form of the variation of outputof a component of this radiometer with a variation in temperature;

[0120] (iii) measuring the temperature of at least one component of theradiometer;

[0121] (iv) combining the one of said stored parameter values with thefunctional form of the component output at the measuring temperature inorder to calculate a compensated radiometer output.

[0122] Defocus

[0123] According to another aspect the invention comprises an imagingradiometer having a focuser adapted in use to focus incident radiationonto a detector feed or a detector feed array of detector feed elementsfor normal imaging operation of the radiometer, and a defocuser adaptedto defocus radiation from the scene that falls onto the detector feed ordetector feed array so that each detector feed element experiencessubstantially the same radiation incident upon it, in use.

[0124] The defocusing element may be the same element as the focussed.The focuser may have a focussing configuration and a defocusingconfiguration. Preferably the focussing element is moved angularly tochange from the focussing to defocusing configurations. The focussingelement may be moved angularly about 90° between the configurations.Preferably the focussing element comprises a linear polariser.Preferably the focussing element focuses radiation with a first linearpolarisation onto the detector array, and not radiation with apolarisation that is displaced by 90°.

[0125] The focussing element may comprise a dish of generally parallelwires. The focussing element may be disposed in front of the detectorfeed array, relative to a scene capturing aperture of the radiometer. Areflector may be provided and/or a polarisation-changing element. Thepolarisation changing element is preferably provided between thereflector and the focussing element. The polarisation changing elementmay be a ¼ plate, such as a meanderline device.

[0126] Active Temperature Stabilisation

[0127] According to another aspect the invention comprises a radiometercomprising a detector; a detector feed coupled to the detector; anamplifier receiving signals from the detector and providing amplifiedsignals to a control processor; one or more temperature sensors sensingthe temperature of one or more temperature-sensitive components of theradiometer for example one or more of the temperature of the radiometeras a whole, the detector, the amplifier, or the detector feed; andtemperature control means; the temperature sensor providing temperaturesignals in use of the radiometer to the control processor indicative ofthe temperature of the one or more temperature sensitive components andthe controller being adapted to control the temperature control means soas to maintain the temperature of the one or more temperature sensitivecomponents substantially constant in use.

[0128] Thus excessive variations in temperature of components whosetemperature fluctuation would affect the gain and/or offset of achannel, or between channels, are avoided, and so excessive temperatureeffects are avoided.

[0129] The temperature sensor and temperature control means provide afeedback system to enable the controller to maintain an appropriateoperational temperature. The temperature control means may comprisethermoelectric, e.g. Peltier effect, components, the control processorcontrolling the electric current to the thermoelectric device inresponse to the temperature signals it receives.

[0130] Individual temperature sensitive elements/components may havetheir own temperature control means and/or temperature sensors.

[0131] Changing Tilt Angle

[0132] According to another aspect the invention comprises a conicalscanning radiometer imager having a reflector inclined at a tilt angleto an axis about which the reflector is adapted to rotate in use, inwhich tilt angle adjustment means is provided adapted to alter the tiltangle of the reflector relative to the axis about which it rotates.

[0133] The tilt angle adjustment means may be operable whilst thereflector is rotating, and/or when the reflector is stationary.

[0134] A controller preferably provides signals, preferably electrical,to tilt angle adjustment means, which may comprise a motor, or asolenoid, a piezoelectric device, or an electromechanical transducer, toadjust the tilt angle in use. A tilt angle sensor may be providedadapted to provide signals indicative of the tilt angle to thecontroller. The tilt angle adjustment means may be adjustable todifferent positions to vary the spacing between the centres ofcones/circles of scene space that optics of the radiometer projects, bitby bit in time, onto respective detector feed channels (to make thecentres of the line of cones closer or further apart). The tilt anglemay be adjustable to superimpose two, or all, cones.

[0135] Pixel Integration

[0136] According to another aspect the invention comprises a radiometerimager having a conical scan pattern in which radiation from a pixel inan observed scene space is directed onto a detector feed for a period oftime when the optics of the scanning mechanism of the imager isappropriately aligned, electrical signals from a detector associatedwith the detector feed being correlated with a point in scene space towhich they correspond by a control processor, and in which theintegration time over which signals resulting from a particular point inspace are allowed to integrate at the detector or control processor iscontrollable by the control processor, so as to provide an imager whichhas a variable pixel integration time.

[0137] Preferably the imager is adapted to control the integration timeof some pixels to be different from other pixels in the image that iscreated. Preferably the at least one or some pixels on a particularconical scan can have different integration times, under the control ofthe control processor, to other pixels on the same conical scan. Thismay be achieved by varying an electronic time gating of receivedradiation so as to keep the gate open for longer, and/or by varying thespeed of rotation of a reflector in the scanning optics. (e.g. a longerintegration time could be achieved by varying the speed of rotation of areflector in the scanning optics). The control processor may be adaptedto determine whether the observed scene, or part of the scene, or anobject in the scene, is moving faster than a predetermined speed, andmay be adapted to alter the pixel integration time to increase it ifmovement above a threshold speed was not established.

[0138] If a scene changes too fast an increased integration time for apixel will not give more data about that pixel in the scene at anyparticular time, but will instead “smear” changing data over time andproduce an averaged value, instead of better resolution: it will not belooking at one pixel in the scene all of the time that the gate is open.

[0139] Pixel selection means may be provided to select which pixels inthe observed scene means would have a modified integration time. Thepixel selection means may be manually operable to select manuallyparticular pixels or groups of pixels (e.g. the user could draw a linearound an area of interest) to have their integration time modified,and/or the imager may have an automatic pixel selection routine adaptedto identify pixels of a certain category or categories and alter theirintegration times.

[0140] Vary Pixel Calibration with Scan Angle

[0141] According to another aspect the invention comprises a method ofcalibrating a radiometer having a detector channel having a detectorfeed and a detector, and a scanner which scans an observed scene bydirecting different parts of the scene onto a detector feed of thedetector channel at different times, with each notional pixel in anobserved scene observed by the channel having an associated scan angle,the method comprising modifying the observed scene temperature detectedby the detector by a calibration or compensation function which isdependant upon the scan angle of the notional pixel being observed.

[0142] Preferably the method comprises having a plurality of detectorchannels and modifying the respective detected scene temperatures byrespective scan angle dependent calibration or modification functions.Each detector channel may have a different scan angle dependentmodification or calibration function. Alternatively more than one, orall, channels may have a common scan angle dependent calibration ormodification to the detected temperature for a scene pixel.

[0143] The calibration or modification function may comprise amodification to the gain and/or offset voltage of a channel for theparticular pixel concerned.

[0144] The method may also comprise compensating for the temperature ofthe channel, or channel components, when evaluating the scenetemperature for a pixel, preferably by modifying the gain and/or offsetvoltage with temperature of the device.

[0145] According to another aspect the invention comprises a scanningimaging radiometer having a scanner which is adapted to scan notionalpixels in an observed scene onto a detector element of a detectorchannel, and a control processor that is adapted to receive signals fromthe detector element and is adapted to evaluate the scene temperature ofthe notional pixel from the output of the detector element and from aconcordance or algorithm which compensates for the scan angle at whichthe pixel was evaluated.

[0146] Preferably the concordance or algorithm also compensate for thetemperature of temperature sensitive components of the radiometer.

[0147] Software/Processor

[0148] According to another aspect the invention comprises a signalprocessor, or software, which when loaded into a radiometer provide aradiometer in accordance with any other apparatus aspect of theinvention, or which when controlling the operation of a radiometercauses the method any method aspect of the invention to be performed.

[0149] The software may be provided on a data carrier, such as a disc.

[0150] The invention has most application to radiometric images, such asmm wave devices, possibly in the range of about 30 GHz to about 300 GHz,but possibly in the infra red range, or other wavelengths.

[0151] Embodiments of the invention will now be described by way ofexample only with reference to the accompanying drawings, of which:

[0152]FIG. 1 shows a graph of detector output voltages against scenetemperature, T_(S), for radiometers having physical temperatures T₁ andT₂;

[0153]FIGS. 2A and 2B show graphs of detector output voltage againsttime for a radiometer at two different temperatures;

[0154]FIG. 3A illustrates schematically a scanning real time imagingradiometer;

[0155]FIG. 3B illustrates in more detail the feed array of theradiometer of FIG. 3A;

[0156]FIGS. 3C to 3G illustrate schematically non-straight detectorarrays;

[0157]FIG. 4 illustrates schematically a staring array imagingradiometer;

[0158]FIG. 5A illustrates schematically the overlap of fields of view ofdifferent channels of the array of FIG. 3 at different scanningpositions of the scanner of the radiometer;

[0159]FIG. 5B illustrates schematically a technique for calibrating orcompensating for temperature fluctuations in the radiometer of FIG. 3;

[0160]FIGS. 6 and 7 show other plots of scan cones over viewed scene;

[0161]FIG. 8 illustrates a free standing calibration technique for afurther embodiment of the invention;

[0162]FIG. 9 illustrates a real-time imaging conical scan quarter waveplate radiometer in accordance with the invention;

[0163]FIG. 9a illustrates schematically the scanning of differentregions of the observed scene into different detector feeds of thedetector feed array of the radiometer of FIG. 9;

[0164]FIGS. 9b and 9 c show a linear array of feed elements and anassociated sensitivity pattern;

[0165]FIGS. 9d and 9 e show an annular array of feed elements and anassociated sensitivity pattern;

[0166]FIGS. 10a and 10 b show the radiometer of FIG. 9 configured forcalibration;

[0167] FIGS. 11 to 13 show another scanning radiometer;

[0168]FIG. 14a shows another radiometer;

[0169]FIG. 14b shows the circular scan pattern transformation to a twodimensional image suitable for presentation on a monitor screen or datastorage ; and

[0170]FIGS. 15A and 15B respectively show variation in gain and offsetwith scan angle θ.

[0171]FIG. 1 illustrates schematically the output voltage V of adetector of a radiometer against scene temperature and illustrates thatwhen the radiometer is at temperature T, the detector has arelationship.

V=g ₁ T _(1S) +V ₀₁ and V=g ₂ T _(2S) +V ₀₂

[0172] When the device is at T₂ (where T₁₅ is the scene temperature withthe device at T₁, g₁ is the gain at T₁, and V₀₁ is an offset voltagewith the device at T₁, and where T_(2S) is the scene temperature withthe device at T₂ g₂ is the gain at T₂ and V₀₂ is an offset voltage withthe device for T₂).

[0173] Thus the radiometer device has a performance that is temperaturedependent. Moreover, the straight line graphs shown in FIG. 1 aresimplistic: the receiver antenna may have a temperature dependentresponse, the amplifier will have a temperature dependent response , andthe detector will have a temperature dependent response. Furthermore,the temperature of the radiometer device in use is unlikely to beexactly the same at all points in its structure; there will be thermalgradients.

[0174]FIGS. 2A and 2B show the same effects. FIG. 2A is a graph ofdetection voltage vs time when a detection channel is observing a sceneat a first point in time when the channel is at temperature T₁, and FIG.2B is a graph of voltage vs time at a later point in time, when thechannel is at temperature T₂. Also shown on FIGS. 2A and 2B are theoffsets V₀₁ and V₀₂ are the voltage offsets at T₁, and T₂, and g₁ and g₂are the gain at T₁ and T₂.

[0175]FIGS. 3A and 3B show a real time mm wave meanderline ¼λ platebased conical scanning radiometer 10. The real time passive scanning mmwave imaging radiometer 10 has a scanner 12, a focussing lens assembly14, an antenna feed array 16, an amplifier 18, a detector 20, amicroprocessor 22, a temperature sensor 24, and an image display 26.

[0176] The scanner 12 comprises a flat or slightly curved reflectorplate 28 rotatably mounted about an axis 30, and inclined at an angle θ(say about 5°) to the normal to the axis 30. The focussing lensarrangement 14 comprises a quarter wave meanderline plate 32 providedbetween a focussing dish 36 and the reflector plate 28. The dish 36comprises a polarising reflector element (e.g. a wire grid).

[0177] Incident radiation 5 a is linearly polarised by the grid/dish 36,which may have wires inclined at 45° to the vertical (say) so that thecomponent of radiation with a plane of polarisation 45° to the vertical(90° from the line of the wires in the grid) is transmitted through thegrid 36. This linearly polarised radiation, referenced 5 b, encountersthe meanderline plate 32. The plate 32 has the fast and slow axes of themeanderlines inclined at 45° to the direction of the wires on the grid36 (and hence to the polarisation of the radiation 5 b). Radiation 5 c,emerging from the meanderline plate 32 is circularly polarised and isreflected from reflector plate 28 as radiation 5 d, which is circularlypolarised in the opposite sense to radiation 5 c. When radiation 5 dencounters the meanderline plate 32 it is converted back to linearlypolarised radiation, radiation 5 e, which has its plane of polarisationrotated by 90° in comparison with radiation 5 b. When radiation 5 eencounters the focusing grid 36 it is reflected and focused onto thefeed array 16.

[0178] An inclination of the plate 28 by θ causes the scanning of anangle of 4θ over the antenna.

[0179] The feed array antenna 16 has two rows, rows 40 and 42, ofdetector elements, or horns 44 (best shown in FIG. 3B). Each horncomprises a detection channel. A single detector element 44 has tracedout on it a circular scan pattern from the observed scene as the plate28 rotates. As the detector elements of a row lie adjacent each otherthe image formed from each row is a series of displaced circles, asshown in FIG. 5A.

[0180] The output of each horn 44 (referenced 46) is fed to theamplifier 18. The amplifier 18 outputs to the detector 20 (e.g. aSchottky detector). The microprocessor 22 receives signals from thedetector 20 and the temperature sensor 24 and processes these signals toproduce an image 48 which is displayed on the display 26.

[0181] The detector array may comprise a feed horn, microwave waveguide,which may have a polarisation direction, and monolithic microwavecircuit (MMIC) receiver and detection circuit. The resolution of theradiometer is preferably diffraction—limited.

[0182] The above is a so-called conical scanning system. It isparticularly compact.

[0183]FIGS. 3C to 3F illustrate that the idea of having more offset feedelements of an array, so as to decrease their effective spacing whenviewed along the direction of radiation incident upon the detectorarray, applies not only to straight, flat, arrays, but also to curvedarrays. There can be more than two lines, or tiers, of feed elements.FIG. 3F shows a circular array modified to use the present invention(i.e. two circles, with offset feed elements). Offsetting feed elements,at different distances from the aperture of the device, allows morespace for each feed element/allows a closer effective spacing.

[0184] Relative Calibration

[0185]FIG. 5A shows several detection paths 50 to 54 traced out on ascene being observed by the detector 20 (or more accurately shows theregions of a scene being observed that are detected by the detector dueto the scanning motion of the plate 28). Circle 50 is the annular areaof the scene projected onto or into a first channel (say channel 50′shown in FIG. 3). Circle 51 is the annular region of the scene detectedby channel 51′ as the scanner scans, circle 52 that is that detected bychannel 52′, and so on. If the paths 50 to 54 are close enough to eachother a good image of the scene can be built up. It will be appreciatedthat by correlating the time of the detected signal with the position ofthe scanner each circle 50 to 54 is actually made of discretemeasurements. This represented in FIG. 5B by what can be considered tobe pixels 56, 57, 58.

[0186]FIG. 5B also shows crossover points 60 to 65 where two circlesintersect. Different circles are detected by different channels. At acrossover point two channels are detecting from the same point in spacein the scene.

[0187] The adjacent (or not necessarily adjacent) channels detect fromthe same crossover point virtually simultaneously, or at least closeenough in time for the temperature of the device not to have changedsignificantly, and close enough for the emitted radiation from thatpoint in the scene not to have changed significantly (let us assume).

[0188] It will be appreciated that the different circles could bedetected by the same channel, but at different times, if the field ofview of the circular scan is changed a little between scans by thescanner. What is achieved is two intersecting scan circles (or paths,not necessarily circles) where the temperature of the device is constant(for one scan relative to the other) and the observed scene is constant(or effectively so).

[0189] For two scan paths detected using the same channel (let us saythat circle 50 and circle 51 are traced out by the scanner 40 ms apart),then at time one:V_(Detector) = Gain_((Channel  1))^((temp  of  device)) × T_((Channel  1))^((of  scene)) + V_(offset(Channel  1))^((temp  of  device))

[0190] with the gain and offset being device-temperature dependent.

[0191] For the case where there are two channels then:

V _(Det Channel 1) =g ₁(t)×T _(scene) +V ₀₁(t)

and

V _(Det Channel 2) =g ₂(t)×T _(scene) +V ₀₂(t)

[0192] There are two crossover points for each pair of adjacent (orrelevant) channels, for example points 60 and 63. The temperature at thescene for these two points can be assumed to be different, so we have:

V _(Det Channel 1 point A) =g ₁(t)×T _(scene A) +V ₀₁(t)  (equation a)

V _(Det Channel 2 point A) =g ₂(t)×T _(scene A) +V ₀₂(t)  (equation b)

V _(Det Channel 1 point B) =g ₁(t)×T _(scene B) +V ₀₁(t)  (equation c)

V _(Det Channel 2 point B) =g ₂(t)×T _(scene B) +V ₀₂(t)  (equation d)

[0193] The scene temperature is measured or known in some way. Forexample, the main factory calibration:

V _(Detector)=gain×T _(scene) +V _(offset)

[0194] has a non-temperature related value for gain and for V_(offset).V_(Detection) is known and so T_(scene) can be evaluated. This is donefor one of the channels, say channel 1, which serves as a main referencechannel against which relative gain and offset of different channels isdetermined. Thus, values for T_(scne A) and T_(scene B) in equations (a)to (d) are known.

[0195] The four equations (a) to (d) now have four unknown variables andare solvable for those variables. Thus, new temperature-sensitive valuesfor g₁ and V₀₁, and for g₂ and V₀₂ are established. They are moreaccurate relative to each other at that time in use than are thefactory-set base line values for g₁ and V₀₁, and g₂ and V₀₂. Thisenhances image quality, and/or avoids the interruption and expense ofreturning the radiometer to a remote site for calibration.

[0196] Similarly, values for g₃, V₀₃ can be established, either byrelative comparison between channel 1 and channel 3, or channel 2 andchannel 3 (channel “already having been relative-calibrated relative tochannel 1” vs new uncalibrated channel).

[0197] Similarly, in FIG. 5A channel 4 crosses channel 1 (and channels2, 3, 5, etc), and its gain and offset can be relatively calibrated.

[0198] As a refinement, since the temperature at a crossover point issubstantially to be the same the system can evaluate two temperaturesfor it using the two crossing channels (and using their factory setabsolute calibration). It is possible to take the average of what thechannels say is the scene temperature and use that as the T_(S) in theprocessing.

[0199] It is believed that the temperature of the focal plane arrayantenna 16 is important in image quality, or more precisely thatvariations in temperature across the focal plane array antenna are thecause of image degradation and compensation/calibration for suchtemperature variations can improve image quality.

[0200] The relative calibration of the channels may take placeperiodically, say every second or so, or every ten seconds or so, withthe device capturing images ever 40 ms or so, or alternatively therelative calibration algorithm could run more often, possibly in theconstruction of each image, or between every 1, 2, 3, 4, 5, 10, 20, 30,40, or more images.

[0201]FIG. 6 shows a conical scanning pattern over a larger angle. Thescanner may scan cones 60, 61, 62, 63, 64, 65, 66 etc in turn.Alternatively, it could scan 60, 62, 61, 63, 64, 66, 65, 67 in turn, orany pattern where adjacent conical scans are scanned close enoughtogether in time for the scene to be taken as not to have changedsignificantly.

[0202]FIG. 7 shows a conical scan pattern where the lower row of scancones intersect the upper row, with each circle/cone of the lower rowintersecting a circle/cone of the upper row at two crossover points.This allows relative calibration between the circles of the upper rowand those of the lower row (as described earlier), and this enhancesimage quality.

[0203] It will also be noted that the image sampling/density of scanlines in the middle of the field of view is higher than at the edges.For a conical scanner with two adjacent linear arrays the imagingsampling is more than Nyquist at the top and bottom of the image (inboth directions). In the centre of the image the sampling is close toNyquist (or just under) in the horizontal direction and Nyquist in thevertical direction (defined by temporal sampling).

[0204] Sometimes a system may be able to over-sample and get moreequations than there are unknown parameters, and the equations may notsolve exactly (the parameters may not work out quite the same dependingupon which groups of equations are solved to determine them). In such acircumstance the parameters (e.g. gain and offset for a particularchannel) may be fitted to something (e.g. averaged, or least squaresfitted). An over-determined problem (because of the scanning) can beused to advantage. It is possible to have more cross-checks to ensuresolutions broadly match and are therefore reliable (and if one equationproduces an anomalous result it could be omitted from the analysis. Itcan also be useful to have redundancy if one (or possibly more) of thechannels fail.

[0205] It will be noted that the technique works best if the twotemperatures have a large temperature difference, which will be if oneis using the upper most and the other the lower most regions of theimage, which will be for adjacent channel overlap. To get the best ofboth worlds it might be good to do the correction with three channels ata time, one central channel and the two adjacent ones. The chance of twoadjacent channels failing is much less than one channel failing.

[0206] It will be appreciated that the conical scanning architecture ofthe real time imager described lends itself admirably to techniqueswhich can improve the relative calibration. This is because many regionsin the image are measured several times by different radiometerchannels. The assumption that the temperature in the image does notchange between measurements is generally valid, and if not, the fact canbe known from examination of the raw uncalibrated data from theradiometers (and the device knows not to use that technique at thatmoment—indeed the device may have this check as part of its relativecalibration operation). This technique can also be implemented onalternative imager architectures such as the pattern from a circulararray of feeds.

[0207] As will be appreciated from the foregoing, a particular channel,a master channel, would be chosen as having a fixed calibration gain andoffset. The neighbouring channel would then have its gain and offsetmodified so that the temperature measured at the points where thechannels cross are the same. For neighbouring channels there are twocross-over points and if the scene temperatures at these two points aredifferent (which is generally the case) the corrected gain and offsetfor the channel can be calculated. If by chance the temperatures at thetwo points are very similar (a fact which can be established byexamining the uncorrected calibrated data) it should be arranged for theimage to view a scene in which the radiation temperature across thescene varies by a large mount. For an outdoor operation, such adirection would be that towards the horizon (sky and land have differenttemperatures). In such a fashion this calibration method would move fromone channel to the next making the relative calibration such a techniquegreatly improves image quality.

[0208] An extension of the above technique is that different cross-overpoints could be used not just the ones on the adjacent channels. Thiscould be used to complement the above method, to improve still furtherimage quality. Furthermore, the angle of the tilt on the imager spinningdisk could be changed to allow the imager to change its field-of-viewand view different regions of a scene or sources for calibrationpurposes.

[0209] Other possibilities for relative calibration are making gain andoffset corrections on neighbouring channels using a 360° conical scanaveraged temperature or AC components of the temperature. The assumptionbeing here that the averaged temperatures or the AC component of thetemperatures on two adjacent channels are the same.

[0210] Typical frequencies for radiation detected by the radiometer maybe 35 GHz, 94 GHz, 140 GHz, 220 GHz; these are the atmospheric windowbands for mm waves.

[0211] The radiometer uses monolithic microwave integrated circuits(MMICs) and has a small volume.

[0212]FIG. 4 shows a staring antenna 70 having a grid of antenna horns72, a temperature detector 74 detecting the temperature of the array, anamplifier 76, and a detector 78.

[0213] Even though FIG. 4 shows a staring array it still performs a scanfor calibration purposes. A microscan could be performed periodically tocause more than one channel to see substantially the same point in thescene at substantially the same time (but not quite the same time). Anoptical component, or a whole imager, is caused in the microscancalibration operation to move slightly (e.g. so that one channelmeasures the vertical and/or horizontal nearest neighbour pixels, aswell as its own pixel). This “dither” creates the same multiple channelslooking at the same temperature (same point at very close times) effectand so the staring array can perform relative calibration betweenchannels. It can, of course, have absolute calibration performed on it.

[0214] In order to get two physically separate parts of the sceneobserved by the same channel in a staring array (and hence two widelydifferent temperatures observed) it may be necessary to move the fieldof view of the radiometer, or do something else to achieve that.

[0215] If it is not possible to observe two different temperatures withboth channels being relatively calibrated, it may be necessary to assurethat only one of the gain and offset varies with temperature and thatthe other is fixed, and calibrate using the one that is assumed to vary.Alternatively, single point calibration may be used. These points applyto scanning radiometers as well.

[0216] Free Standing Calibration

[0217] The use of crossover points in different scans from differentchannels to perform a relative calibration between channels assistscalibrating one channel relative to another, but it does not assist insetting the absolute calibration of the “master” channel (or indeedabsolute calibration of any channel).

[0218] It is envisaged that the device 10 may return to a laboratory orfactory periodically (e.g. every three or four months) for resetting ofthe absolute calibration of the channels.

[0219] In addition to this, or possibly instead of this, the device maybe capable of adjusting its absolute calibration in the field.

[0220] The device 10 is tested in a factory or laboratory prior torelease to a customer with the device at a large number of temperaturesand a performance look-up table or algorithm is constructed correlatingdevice temperature with gain and offset voltage. Thus, for each devicetemperature a corresponding value for gain and offset voltage isestablished by the device in use. This is typically a look-up table oralgorithm in the processor 22.

[0221] This technique of correcting the device gain and offset fordevice temperature can be applied to devices which have relative channelto channel calibration, and to those that do not.

[0222] It will be appreciated that controlling the temperature of thewhole device to be at a known, same, temperature and establishing thedevice gain and offset for the device at that temperature is a littlesimplistic (but better than not allowing for device temperature changesin the field). In practice in the field, with the device in use, thereare likely to be temperature gradients over the device: one channel maybe at a different temperature to a remote channel; the amplifier may beat a different temperature to the antenna, the detector may be at afurther different temperature.

[0223] It is believed that the main temperature sensitive components arethe rf amplifier (with perhaps about ¾ of the temperature dependenteffect from that) and the detector (with perhaps ¼ of the temperaturedependent effect from that). In practice it is possible for theamplifier and the detector to be close together so that only onetemperature sensor is needed.

[0224] Also possible, but perhaps more complicated to implement, is anarrangement where temperature detectors for more than one place in thedevice are provided. FIG. 3 shows only one temperature sensor 24, butthe amplifier and/or detector could have either a single temperaturesensor or their own temperature sensor(s). Groups of horns, or eachhorn, could possibly have an associated temperature detector. Themicroprocessor 22 would then receive a plurality of temperature signalsfrom a plurality of regions/parts of the device.

[0225] The microprocessor 22 could have a matrix/look-up table relatingthe temperature sensor temperature to a gain and offset, or relatingdifferent and all combinations of sensor temperatures to an associatedoverall device gain and offset for each channel. This would, of course,need to be established in advance of use of the device by operating thedevice at these combinations of temperatures to observe known scenetemperatures (at least 2) and working out the appropriate gain andoffset to load into the look-up table (or to create an appropriatealgorithm).

[0226] An alternative approach is to determine the effect or gain andoffset of changes in temperature of one or more individual components ofthe device (or groups of components) and to generate the overall gainand offset from those. For example each channel (or a group of adjacentchannels) may be tested in the factory to determine how their outputsignal (S_(channel)) varies with temperature of the channel (T_(c)) anda function S_(c)(T_(c)) created (either look-up or algorithm-based).

[0227] How the amplifier performance (S_(A)) varies with amplifiertemperature (T_(A)) can be determined to create a function S_(A)(T_(A)).The detector could be evaluated at different detector temperatures todetermine how the detector voltage (V_(D)) varies as a function oftemperature of the detector (T_(D)) to created V_(D)(T_(D)). The outputvoltage from the detector is then predicted to be related to:

V _(output) αS _(C)(T _(C))×S _(A)(T _(A))×V _(D)(T _(D))

[0228] and a compensated output voltage (V_(calibrated)) can beobtained:

V _(calibrated) =V _(output)ƒ(T _(C))ƒ(T _(A))ƒ(T _(D)).

[0229] Expressed another way, the gain and offset voltage can be:

gain_((channel)) =g _(absolute calibration)×ƒ(T _(C))ƒ(T _(A))ƒ(T _(D))and

V _(offset (channel)) =V _(offset (absolute calibration))×ƒ(T _(C))ƒ(T_(A))ƒ(T _(D)).

[0230] The ƒ(T_(C)), ƒ(T_(A)) and ƒ(T_(D)) are evaluated/created byoperating the device at a range of temperatures for the parametersT_(C), T_(A), T_(D), using reference temperature sources. It may bepossible to predict one or more of ƒ(T_(C)), ƒ(T_(A)) or ƒ(T_(D))without physically testing the apparatus to create/determine them.

[0231] Just as the embodiment of FIG. 1 may have several temperaturesensors, so may the embodiment of FIG. 4 (again, possibly a temperaturesensor for the amplifier and/or detector, and one for each channel, oreach group of channels).

[0232] Antenna Design:

[0233] A further feature of the embodiment of FIG. 3, and one that isapplicable to other mm imaging radiometers, especially real timeimaging, is the design of its antenna. A conventional antenna has asingle row of horns and is a certain length (in order to get the desiredsize of scene image scanned).

[0234] It has been appreciated that the current spacing ofhorns/channels in a single row of horns is not sufficient to achieveNyquist sampling in the image plane, thereby introducing the possibilityof aliasing in the constructed image: the channels are namely are toofar apart. Yet they have to be a certain size to capture the radiationof desired wavelength.

[0235] The solution to this problem is to have a second (or further) rowof horns/channels in substantially the same plane as the first row, butwith the horns of the second row offset or staggered in the elongatedirection of the row. This then essentially means that the spacing ofthe horns in the elongate direction is halved (for two rows). Withreference to FIG. 3, the centre of horns 50′ and 51′ are a distance Dapart, but the distance apart in the elongate direction of the antennabetween horn 50 of row 40 and horn 53′ of row 42 is D/2. This enables usto get the horns closer together, effectively, and allows us to approacha Nyquist sampling distance, thereby avoiding aliasing in theconstructed image. We may use three, four, or more rows ofchannels/horns to reduce their effective separation in the directiontransverse to the scanning direction (the scanner 14 scans the observedscene over the antenna 16 in a direction perpendicular to the elongatedirection of the antenna).

[0236] The offset of the detector channels does not have to be halfperiod, but that achieves the best resolution when there are only tworows. For three rows the offset could be ⅓ period (1/n period for nrows).

[0237]FIG. 9 shows another real time imaging scanning radiometer 90 in aquarter wave plate, e.g. meanderline, scanner. The radiometer 90 has afocal plane detector array 91, a polarisation-sensitive selectivelytransmissive or reflective grid or dish 92, referred to as a focusingreflector (this could be a grid of parallel wires), an angled, ortilted, rotating reflector plate or disc 93, and a quarter wave plate94. A meanderline plate is a known device which is essentially a quarterwave plate and which has a series of meandering lines with repeatingcomponents in orthogonal directions which have the effect, dependantupon the plane of polarisation of incident radiation and the orientationof the meanderline plate relative to it, changing the polarisation fromlinear to circular as it passes through the meanderline plate 94. Thequarter wave plate 94 is for normal imaging operation arranged with itsfast axis at 45° to the plane of polarisation of the radiation thatreaches it from the gird 92 (45° to the plane of polarisation of theradiation 96 a shown in FIG. 9). The quarter wave plate 94 and thespinning disk 93 act together as a twist reflector, enabling element 92to focus incoming radiation 96 whilst providing a conical scan of theimage.

[0238] In normal imaging use, shown in FIG. 9, thermal radiation 96 froman observed scene enters the radiometer and passes through the grid ofparallel wires that make up the reflector 92 if the radiation ispolarised with its electric field vector perpendicular to the directionof the wires. Similarly, radiation which is polarised with its electricfield vector parallel direction of the wires is reflected.

[0239] The polarised component of the incoming radiation 96 is 96 a. Thelinearly polarised radiation 96 a passes through the quarter waveplate94 and is transformed to the right hand circular polarisation (RHC)radiation 96 b. This only happens when the fast (or slow) axis of themeanderline plate, or quarter wave, is inclined at 45° to the plane ofpolarisation of the radiation 96 a. (as mentioned earlier). The RHCradiation then reflects off the plate 93 and becomes left hand circularpolarisation radiation (LHC) and, passes back through the quarter waveplate 94 and is converted from LHC radiation to linearly polarisedradiation 96 c, but with its plane of polarisation at 90° to that ofradiation 96 a. This is to say radiation 96 c is now orthogonallypolarised relative to radiation 96 a, and to the dish 92.

[0240] The radiation 96 c then reflects off curved reflector dish 92(since its plane of polarisation is now at 90° to what it was when itcame in originally) and is focused onto the focal plane detector array91. This is the normal imaging operation. It will be recalled that thereceiver feed horns of the array 91 have a polarisation plane, whichshall be assumed to be parallel to that of the radiation receiving it,after the optics discussed above.

[0241] The focal plane array 91 could be a linear, or curved, or anannular array of detector feed elements/radiometer receivers.

[0242]FIG. 9b shows a linear array of receiver feed elements, orchannels, in an image plane. FIG. 9c shows that the array of FIG. 9bgives linearly displaced circles of sensitivity in the imager field ofview.

[0243]FIG. 9d shows an annular array of receiver feed elements, orchannels in the focal plane, which gives rise to an annularly displacedset of circles of sensitivity in the imager field of view, shown in FIG.9e. The centres of the circles of sensitivity of the imager field ofview shown in FIGS. 9c and 9 e are placed either in a line or circle,depending if they are associated with the linear array of FIG. 9b or theannular array of FIG. 9c.

[0244] The radiometer also has a defocused mode. If the dish reflector92 is rotated by 90°0 in comparison with its orientation in thediscussion above then the radiation entering the detector array 91 isunfocused, and so will be substantially the same for all channels in thedetector horn array 91. This is because in this configuration thepolarisation to which the detector horn array is sensitive radiation canpass directly through the grid 92 without passing through any of thescanning optics (components 94 and 93) and hence without being focussed.

[0245] Referring to FIG. 10a, a thermal radiation source 99 a, a hotsource, and another radiation thermal source 99 b, a cold source, areplaced diametrically opposite each other at the periphery of thedetector array 91. These sources can be used for the purposes ofcalibration. As will be described later, rotation of quarter waveplate94 by 45° (from the normal imaging mode) causes radiation from thesesources to enter the channels of the array 91. The fast (or slow) axisof the quarter waveplate is now parallel to the direction of the wiresin grid 92. In this calibration configuration, as the disk 93 rotates anannular image of the detector array 91 is traced out in the focal plane.This means each detector channel gets to measure alternately theemission from the hot and cold calibration sources.

[0246] Looking at FIG. 10a radiation 100 a is emitted by a source 99 aor 99 b. Linearly polarised radiation 100 b is reflected back from dish92 (and orthogonally polarised radiation passes through the dish), theradiation 100 b has its plane of polarisation parallel to one of thefast or slow axes of the quarter wave plate meanderline 94 and passesthrough it, still linearly polarised, is reflected from rotating disc 93as radiation 100 c, (still linearly polarised), passes back through themeanderline plate 94, re-encounters the dish 92, and is reflected andfocussed by the dish 92 (radiation 100 d) to a point 102 at theperiphery of the array 91. It will be noted from FIG. 10a that theradiation 100 a reflected from the dish 92 is a nearly parallel beam(because the source 99 a or 99 b is nearly at the focal point of thedish 92), and that the thermal emissions from quite a large angularrange are collected by the dish 92, as the disk 93 rotates. It will alsobe appreciated that the effect of arrangement of FIG. 10a is that theoutermost two (or perhaps more) channels 103 a and 103 b of the arraysee a known temperature, the thermal source temperature, and that thiscan be used for absolute calibration of those channels, which may beused as a reference channel, or channels, for the relative calibrationof other channels.

[0247] The relevant factor in operating the radiometer in a defocusedmode is the relative orientation of the wires in the grid polariser 92with respect to the plane of polarisation to which the array detectorsare sensitive. Rotating the grid polariser 92 by 90° (from its normalmode of imager mode of operation) changes the system from one thatfocuses to one that receives unfocussed radiation.

[0248] The rotation of the tilted disc 93 causes the point adjacent thefocal plane array that is focused onto the array 91 to trace out theannulus 91 a of varying diameters in the focal plane.

[0249] The thermal source 98 may initially be a hot or a cold loadplaced adjacent to the array 91, and in a second calibration operationit is the other of the hot or cold loads. A single thermal source may beprovided, possibly of changeable temperatures, or two differenttemperature sources, as shown in FIGS. 10a and b.

[0250] The thermal load could be a true thermal load, emission ofthermal radiation being from thermal absorbers at the two differenttemperatures. The temperature of the thermal source 99 a or 99 b couldbe liquid nitrogen temperature, or another defined temperature. Theycould range to a few hundred Kelvin. It is best if the temperature ofthe source 98 is known. There may be a temperature sensor e.g.thermistor to measure the temperature of the thermal source 99 a or 99b, or it may be known in some other way (e.g. the boiling point ofliquid nitrogen at atmospheric pressure is known).

[0251] The thermal source may be manually mountable and/or demountable,or it may comprise a receptacle provided adjacent the focal plane arraycapable of holding a source of hot or cold loads.

[0252] The hot and cold loads could alternatively be produced bythermoelectric e.g. solid state sources, such as Peltier effect devices.

[0253] It will be appreciated that being able to present a sourcetemperature to more than one channel in the array 91 enables relativechannel calibration as previously discussed (whether that be bydefocusing and using the scene as a constant temperature, or whetherthat be using the source 98 as the same constant temperature).

[0254] In addition, if the absolute temperature of the source 99 isknown with sufficient accuracy (e.g. to 1K or so) then that knowledgecan be used to perform an absolute calibration of a channel, and eachchannel of the whole system. For example, the radiometer 90 system couldhave an absolute calibration with a single source 99 of knowntemperature, or with two known (but different) source temperatures,periodically, say daily, weekly, monthly. The system could then performrelative calibration much more frequently (e.g. of the order of everyfew seconds, or as discussed earlier). The source 99 or sources 99 arein the embodiment of FIG. 9 presented to each channel in the system.

[0255] The scanner configuration shown in FIGS. 9 and 10 has low opticslosses, in the example about 0.5 dB. This compares well with the lossesof the scanner in UK Patent Application No. 9707654.1 which has lossesof around 4dB. Thus, the system of FIGS. 9 and 10 is at least twice assensitive as that of GB 9707654.1.

[0256] It will be noted that in the arrangement shown in FIG. 10b thechannels of the detector array need electronic communication to theprocessor or controller that processes their signals, and that this isprovided by electric cables 104 which extend out along a diameter of thedevice. Thus the cables obscure part of the radiation incident upon thedevice. The addition of the hot and cold sources 99 a and 99 b on thesame line as the exit areas of the array's cables does not obscuresubstantially any more of the operational area of the imager than isalready obscured by the cables 104. Thus introducing the source 99 doesnot significantly reduce the optical performance of the device.

[0257] A further advance of this system is that the desired radiationbandwidth is not restricted by the ferrite as in the scanner of GB9707654.1, which is a ferrite-based scanner. This means the bandwidtharound 30 GHz can be from 26 GHz to 40 GHz. This is a bandwidth of threetimes that of the scanner of GB 9707654.1, which is only 5 GHz. Thisoffers a root three, ˜1.7, factor in improvement in the sensitivity.Furthermore, this system could be used at the 94 GHz window. It couldalso be used at the higher frequencies of 140 GHz, 220 GHz and beyondwhere it may be difficult to find a suitable ferrite. It has a much moreflexible operational range of frequencies.

[0258] It will also be appreciated that the angular movement of thereflector 92 and/or the meanderline plate 94 could be effected manually,or the radiometer may have motors to achieve this, under control of acontroller. The user may be able to initiate a calibration operation byinputting a command.

[0259]FIG. 11 shows another conical imaging scanner 110 having aCassegrain configuration. A primary reflector 112 reflects and focusesradiation onto a subreflector 114 which reflects and assists infocussing the radiation into a horn 116, or a focal plan horn array(similar to that shown in FIG. 3).

[0260] When the horn is a rectangular waveguide horn, the apparatus isinherently linearly polarised because of the rectangular horn andwaveguide.

[0261] It is possible to adjust the polametric sensitivity of the systemby placing a linear polariser in the path of the radiation—for examplein front of the horn 116, or in front of the whole imaging scanner(possibly as part of the scanner). The linear polariser could behalf-wave plate. If it is placed before the horn it can be physicallysmall, whereas if it is before the radiation reaches the primaryreflector 112 it needs to be bigger.

[0262]FIG. 12 shows a half-wave plate 118. This routes the plane ofpolarisation of incoming linearly polarised radiation 119 through anangle that is twice the angle between the rectangular horn E directionand the optical axis of half-wave plate. Thus, if φ is 45° the half-waveplate 118 rotates the plane of polarisation by 90°. FIG. 12 shows horn116 having a vertical polarisation and incoming horizontally polarisedradiation 119.

[0263] Thus by rotating the half-wave plate 118 (whether that bedisposed immediately before the horn or elsewhere) it is possible toalter the radiation entering the horn. In the case of FIG. 12 the plate118 can effectively switch off external radiation entering thewave-guide associated with the horn (because the horn has a place ofpolarisation and the plate 118 can be orthogonally crossed). In the caseof FIG. 13, angular displacement of the plate can cause the focussingeffect of the focussing dish 119 to be lost, effectively defocusing thedevice.

[0264]FIG. 14a shows an arrangement for absolute calibration in which aradiometer 140 has a detector 141, a cavity 142, a switch 143, and anantenna 144. When the switch is in an open position radiation from theoutside world propagates directly through the cavity 142 and into theradiometer body to be detected by the detector 141. This is the normalimaging arrangement. The radiometer also has a calibration configurationor mode in which the switch 143 is closed and the radiation that entersthe radiometer body is that from the cavity. As the temperature of thecavity can be measured (the machine has a temperature sensor 145) aknown thermal temperature profile of radiation will be incident upon thedetector.

[0265] The architecture of this system may at first glance appearsimilar to that of the Dicke radiometer which was a switch to compensatefor changes in noise temperature, but the present system has a slowerswitch to calibrate for variation in radiometer device temperature (notnoise temperature), to correct the gain and/or offset voltage. Thecavity 142 could be a length of low loss waveguide. The switch 143 canbe a pin diode.

[0266] The radiometer may have part of its temperature stabilised byinsulation and/or a heating/cooling system. For example, the front end(antenna 144, switch 143, and waveguide cavity 142) may betemperature-stabilised.

[0267] As an alternative to a true thermal alternative source signal(e.g. from the cavity 143), it is possible to provide other referencetemperature signals, selectively using an appropriate switch. Suchalternative source signals could include known magnitude signals (e.g.from an RF source). Alternative source signals at two different levelswould usually be provided at different times, to enable both gain andoffset to be evaluated. A single level could be used if the gain andoffset can be limited to a single parameter, e.g. temperature.

[0268] Active Temperature Stabilisation

[0269] An alternative approach to compensating for temperature changesin use within the imager device is not to allow the temperature of thedevice to change (or at least not to allow the temperature of one ormore critical components to change). Heating/cooling temperature controlequipment (e.g. Peltier effect thermoelectric devices) can be used tomaintain the temperature of some or several or all temperature sensitivecomponents of the imager.

[0270] The design of the imager can be such as to have temperaturesensitive components associated with thermal reservoirs of high thermalmass so that their temperature does not change rapidly. For example, thedetector or detector array can be mounted on a large metal block.

[0271] Scanning vs Staring

[0272] It is preferred to use a scanning imager rather than a staringarray because a staring array is more expensive. If the cost differencereduces, or is worth paying, a staring array may be preferred. With astaring array there is the advantage that one can look at pixels forlonger and this makes the noise smaller, and that can be advantageous.However, there are some disadvantages to staring arrays and these arethat one cannot get Nyquist sampling unless microscan is performed,sampling in-between the pixels. Furthermore, once you start scanningthen you can also get relative calibration. So in actual fact scanningis not so bad, and in spite of the fact the noise is more as you are notlooking at each pixel for very long it is good enough for manyapplications.

[0273] A system with a set of gain an offset coefficients for each pointin the scan is envisaged. Again these could be established by testingthe device in-factory.

[0274] De-Focussing

[0275] A further technique applicable to the embodiments previouslydescribed and also to other devices, is that of de-focussing theobserved scene so as to homogenise the signal received by each channel.

[0276] It can be desirable to do this for a number of reasons. Onereason is when performing an absolute calibration of the channels, usinga hot and a cold source of known radiation temperatures, a bettercalibration of multiple channels is achieved if the channels receive asignal of the same strength. By de-focussing the received radiationdifferences in received radiation strength between different channelscan be minimised or avoided during the absolute calibration process.

[0277] De-focussing the radiation received by channels also has apurpose in relative calibration. As part of the relative calibrationtechnique discussed a comparison of two channels at their crossoverpoints has been discussed because the channels see the same temperatureat that point. De-focussing also means that each channel seessubstantially the same temperature. Thus de-focussing the channels andthen establishing the relative calibration between channels hasadvantages. It also allows for relative calibration of systems which donot have crossover points (e.g. staring arrays). De-focussing to achievethe same scene temperature for each channel for relative calibration maybe performed instead of, or as well as, crossover point comparison.

[0278] The image is, of course, re-focussed for the image datacollection process.

[0279] In the passive real time mm wavelength imagers with which we areprimarily concerned, there is a particularly attractive way of achievinga de-focus. The radiation received is typically polarised, and thisallows the use of polarisation effects to change the apparatus fromfocussing to de-focussing.

[0280] For example, in the ferrite conical scanner embodiment of FIG. 3if the polarising grid 32 is rotated from its focussing position shownin FIG. 3 by 90° so that instead of reflecting beam 5 d it transmits it,the scanner effectively becomes simply a reflector instead of afocussing reflector.

[0281] It is mechanically simple to rotate by 90° a planar member, forexample the polarising grid 32 mounted at its periphery to a supportingdish.

[0282] Alternatively or additionally, the grid 36 could be rotated toachieve de-focus. This may be more difficult mechanically, but stillachievable. Both grid 32 and grid 36 could be rotated to achieve thesame effect (e.g. 45° each). There needs to be a change in the relativeorientation of polarisation of the grid 32 and the grid 36,substantially 90° being best.

[0283] It will be appreciated that the defocusing technique will work ona ferrite conical scanner and a meanderline conical scanner by rotatingthe polarising grid facing the receiver feed horns. It could also beachieved by a polarisation switch in the horn feed by flipping thereceived polarisation by 90°.

[0284] Once the apparatus is defocused then the assumption is that allchannels measure the same temperature. All radiometers in the array canthen be used to measure the temperature and then calculate the averagetemperature of the defocused bland scene. In this way one would ensurethat the uncertainty on the absolute level of the temperature would beminimised. This can be important for absolute calibration.

[0285] In some cases it may not be completely true that the radiationtemperature over the whole of the bland scene is the same. This may bein security imaging applications where things are very close to theimager. In this case it is possible to make the assumption that adjacentchannels are measuring the same temperature. Thus, in the discussionwhich says that two channels may observe the same point in the scene itis also intended to cover observing nearly the same point close enoughso that it is reasonable to assume that the channels are observing thesame temperature, or nearly so.

[0286] It may also be possible to slightly defocus by moving the“optics” in the axial direction (relative movement of the focusing dishand the horn array). In this case it is also possible to make theassumption that adjacent channels are measuring the same temperature.

[0287] To perform the calibration using a single scene temperature, onecould solve the 4 equations for the 4 unknowns by performing thedefocusing technique on two different radiation temperatures.

[0288] Redundancy in detection channels that contribute to an image canbe very helpful.

[0289] Round-up errors can be minimised by averaging values, and it alsoallows the system to compensate for failed detectors (e.g. by usingsignals from working detectors). It also allows for the comparison ofthe evaluated temperature, as evaluated by more than one channel, so asto identify significantly different estimated temperatures and henceidentify potentially failed detectors and/or channels.

[0290] As many mm wave imaging systems are polarisation sensitivedefocusing can also be achieved by the use of a polarisation switch. Ifa polarisation switch is provided directly in front of the detector theradiation being detected would be unfocused. The polariser could be ameanderline arrangement, or a ferrite in fundamental waveguide in thefeed. FIG. 12 illustrates this idea.

[0291] A further feature which may be provided is the ability to use achange in polarisation (or other switch mechanism) to switch the signalsincident upon the focal plane receiver feed array from being signalsfrom the observed scene to being from a reference calibration scene. Themeanderline plate rotation discussed in relation to FIG. 10 is anexample of this.

[0292] Single Point Calibration

[0293] Another technique which may be employed is a single pointrelative calibration. This is applicable to absolute calibration and torelative calibration techniques. Traditionally, it is necessary tocalibrate two channels for gain and offset by looking at two temperaturesources at two different, and known, temperatures. If the temperature ofthe device is known or is estimated there is effectively one onlyunknown in the calibration equation instead of two. This means thatgiven an absolute calibration (in-factory) relative calibrations betweenchannels can be performed using emissions from a single source of knowntemperature.

[0294] In single point calibration it is desired to obtain a relativecalibration when one can only present to the channels a singletemperature. If one only has a single temperature then one only has twoequations for four unknowns. The single point calibration makes use ofthe assumption that says the two unknowns, the gain and offset, are nottruly independent. There is in fact a link. Once this is assumed thereis only really one unknown for a single channel. The assumption is tosay that the thing that links the gain and the offset is the temperatureof the system. This is an assumption of course, but a reasonable one.This is because most changes in gain and offset are due to temperaturechanges. This means one needs to measure the temperature of the system.Then the temperature, the link between the gain and the offset, is knownand it is possible to look up the gain and offset relation, and thenthere is one unknown. So for the single temperature one has twoequations and two unknowns.

[0295] To use the single point calibration one needs to be sure twoadjacent channels have substantially the same input. One way of ensuringthis is to defocus.

[0296] This single point relative calibration technique can be used withor without the de-focussing technique discussed earlier. The singlepoint calibration technique could be used with the sky, for example, asthe single source (or the ground).

[0297] The device may have a dedicated channel directed at the singlesource. This may avoid having to point the entire device at thereference source. For example FIG. 4 shows a dedicated horn 79 directedat the sky. Not having to point the detector array at the referencecalibration source means that it is possible to calibrate frequently,for example for each image, or periodically every few, or tens, ofimages.

[0298] It will be appreciated that the calibration of the radiometerwill be predominantly in software. This means that the design of theradiometer hardware can be kept robust and simple, and the software canbe upgraded as more sophisticated refinements are developed.

[0299] Calibrating Each Pixel in an Image Obtained by a Channel

[0300]FIG. 14 shows schematically a scene 140 that has been notionallypixellated into pixels 142. Annular scan pattern 144 is the part of theobserved scene that is projected, portion by portion over time, onto asingle channel feed in the radiometer of FIG. 9 as the inclined disc 93rotates. Annular scan pattern 146 is the part of the observed scene thatis projected in serial sequence onto a second channel feed in theradiometer of FIG. 9 (and as shown in FIG. 9a). Each pixel in the scanpath 142 may have a different gain and offset associated with them, forthe same channel, due to geometric effects of the instrument. Thusalthough we have previously discussed a single channel having, at anyinstant, a single gain and offset with temperature dispandencies, andindeed that is true to a very good approximation, there may be, at amore detailed level, a variation of gain and offset for a channel withscan angle—the angle θ around the scan.

[0301]FIGS. 15a and 15 b show this. Indeed, there may be two significant“blips” in the gain and offset profiles with angle around the scan (θ),about 180° apart. These may be associated with the base plate which holdthe detectors and electrical wires 104 leading to the detector array,which are at 180° diametrically opposed positions.

[0302] In order to compensate this effect each angle around the scan,scan angle θ, may have its own associated gain and offset for eachchannel. These may be factory set (and indeed will usually initially befactory-set). However, as discussed earlier, gain and offset aredependant upon the temperature of the instrument. The gain and offsetfor each scan angle will also be temperature dependent to some degree.Thus for a conical scanner:

V _((T)) =g(θ, T)+V _(o)(θ, T)

[0303] When θ is the angle around the scan, and T is the temperature ofthe radiometer (or at least the temperature of a criticaltemperature-dependant component).

[0304] Absolute calibration of the gain and/or offset of a channel atscan angle θ may be provided. A plurality of absolute calibration of thegains and/or offsets of a channel at different scan angles θ may beprovided. Preferably absolute calibration at substantially all values ofθ is provided.

[0305] It will be appreciated that detecting and compensating forvariations in gain and offset of a channel with θ can be achieved bytime gating the detected signals so that a “window” opens electronicallywhen the scan angle is at a particular θ, so that the detectoressentially only sees scene information at that angle θ. Moving thetiming of the gating, relative to the top point of the scan changes thescan angle θ being observed.

[0306] It is also desirable to perform relative calibration between twochannels with allowance for variations in gain and offset with scanangle θ around the scan. This may be complicated at cross-over pointsbecause the scan angle for one channel at the cross-over point may beθ₁, but the scan angle at the cross-over point/region for the otherchannel will be θ₂—a different angle.

[0307] One solution is to determine the variation of gain and offsetwith scan angle in the factory/laboratory for each channel and to modifythe factory settings with relative gain and relative offset valuesdetermined in situo, in use of the radiometer, with the relative gainand relative offset compensating for inter-channel variations withtemperature being scan angle independent—i.e. the same relative channelcompensation for temperature variation irrespective of scan angle, butsensitive to the temperature of the instrument.

[0308] An alternative is the other way around: take what has beendiscussed previously relating to relative calibration (or free standingcalibration or absolute calibration) and overlay a modification to gainand offset that is dependant upon the scan angle, but is the same foreach channel—i.e. gain (g)→gain+g (θ)

And V offset→V offset+V _(o)(θ)

[0309] A further possibility is to have a set of scan angle dependantmodifications for gain and offset for each channel, each modificationfor each channel being temperature dependent and known.

[0310] Applying More than One Calibration Technique

[0311] It is envisaged that in one embodiment free standing calibration,and/or absolute calibration and/or relative calibration could beemployed, in any combination, and possibly all three. Free standingcalibration may be used before relative calibration. The frequency ofrelative calibration of the channels may be the same as, or more than,or less than, that of free standing calibration of the channels.

[0312] Changing Tilt Angle of a Conical Scanner

[0313] In a scanner of the kind shown in FIG. 9, where a reflector plateinclined relative to the normal to its axis of rotation is provided,there can be benefits in being able to vary the angle of inclination, ortilt angle, of the reflector plate.

[0314] It will be appreciated that the angle φ controls how large is thescan pattern of the imager (for a tilt angle φ the full width of thescanning cone angle is 4φ).

[0315] If it were desired to collect information about a central lineregion in the observed scene (e.g. detail along a horizon, or thecentral regions of the image) and greater sensitivity were requiredthere, the tilt angle φ could be reduced, thus reducing the conical scanangle. This would increase the amount of data collected along a centralline in the image and raise the signal noise ratio in this part of theimage.

[0316] Conversely, if a wider field of view were required increasing thetilt angle φ would increase the scan angle, at a cost of reducedsensitivity.

[0317] Internal Coating

[0318] It is envisaged that the internal surfaces of the radiometercould be coated with an energy absorbing coating (absorbing at least inthe wavelengths being use for imaging/detection in the radiometer). Thisprevents spurious radiation spoiling the relationship between incidentscene radiation and radiation detected by the detector array. It alsohelps. to reduce the variation of gain and offset with respect to scanangle.

[0319] Pixel Integration

[0320] For normal operation of the mm wave imager of FIG. 9 theradiometer operates at 25 HZ (i.e. 25 “constructed” frames per second ofthe scene, built from conical scan paths which substantially fill theregion of the scene being observed).

[0321] In order to increase the signal to noise ratio when searching fora particular object in the scene, or when looking at a particular regionof the observed scene covered by the field of view of the radiometer,the integration time for certain selected pixels of the observed scenecould be increased, for example from say a few tens of microseconds toseconds.

[0322] This would normally be performed when it was known, or possiblyestablished by the control processor, that the object was not movingrelative to the image scene (at least not laterally or vertically).

[0323] The area of the observed scene that is looked at for longer(greater integration time) could be determined by the human operator, oralternatively by software (e.g. pattern recognition software couldidentify a known object, or a suspected object, and the controlprocessor could increase the integration time of the pixels in the areaof the object). The degree of increase in pixel integration may be user,or computer, controlled, for example over a range of tens ofmicroseconds to seconds.

[0324] It will be appreciated that the various inventions disclosed canbe used with each other in any combination, as can the subsidiaryfeatures of one invention be used with other inventions. Protection forsuch combinations is sought.

1. A method of improving the image quality of a multi-channel imagingradiometer having at least a first channel which detects radiation usinga first detector and a second channel which detects radiation using asecond detector, the method comprising modifying the gain and/or offsetused in the scene-temperature vs detector voltage equations for thefirst and second channels using values for the gain and offset derivedby performing a channel calibration operation, and in that channelcalibration operation ensuring that the first and second channelsobserve substantially the same temperature and using the outputs fromthe first and second detectors in the calibration operation to produce amodified value for the gain and/or offset for the first and/or secondchannel; the radiometer using the modified gain and/or offset for thefirst and/or second channel to create an image using the first andsecond channels.
 2. A method according to claim 1 in which the first andsecond channels observe substantially the same scene temperature.
 3. Amethod according to claim 1 or claim 2 in which a third channel, orfurther channels, has its or their gain and offset calibrated relativeto the first or second channel by use of a channel calibration operationin which two channels observe substantially the same temperature and thedetector signals from them are used to calibrate the gain and offset ofone channel relative to the other.
 4. A method according to anypreceding claim in which one channel has the gain and/or offset of theother channel(s) set relative to it as a reference channel.
 5. A methodaccording to any preceding claim in which both the gain and offset of achannel is modified in the relative calibration operation.
 6. A methodaccording to any preceding claim in which the two channels observe thesame temperature in the calibration operation by observing substantiallythe same point in space in the scene being observed at substantially thesame time.
 7. A method according to any preceding claim which comprisesobserving with the first and second channels a first temperature, thesame temperature for each channel, and a second temperature, the sametemperature for each channel, the second temperature being differentfrom the first temperature.
 8. A method according to any preceding claimin which relative calibration operation determines the channel 1 andchannel 2 gain and/or offset using the equations: (i) Voltage ofDetector 1=gain of channel 1×observed Temperature (channel 1)+Voltageoffset for channel 1 and (ii) Voltage of Detector 2=gain of channel2×observed temperature (channel 2)+Voltage offset for channel 2 todetermine one or more, or all, of: gain of channel 1, gain of channel 2,offset of channel 1, and offset of channel
 2. 9. A method according toclaim 8 in which the obtaining of values for use in equations (i) and(ii) is repeated for two different temperatures, observed temperature 1and observed temperature 2, giving four equations: V_(Det Channel 1 point A) =g ₁(t)×T _(observed A) +V ₀₁(t)  (equation a)V_(Det Channel 2 point A) =g ₂(t)×T _(observed A) +V ₀₂(t)  (equation b)V_(Det Channel 1 point B) =g ₁(t)×T _(observed B) +V ₀₁(t)  (equation c)V_(Det Channel 2 point B) =g ₂(t)×T _(observed B) +V ₀₂(t)  (equation d)and the method comprises solving the equations to determine the fourunknown variables g₁, V₀₁, g₂, V₀₂, with T_(observed A) andT_(observed B), and the detector voltages known.
 10. A method accordingto any preceding claim comprising scanning a first track or path of anobserved scene onto the first channel and scanning a second track orpath of an observed scene onto the second channel, and intersecting thepaths.
 11. A method according to claim 10 in which the two tracks arecaused to intersect at at least two spaced apart points in the observedscene.
 12. A method according to claim 2 or to any preceding claimdepending directly or indirectly from claim 2, comprising checking thepre-modification values for the temperature being observed by the firstand second channels to ensure that they are close enough to each otherto be considered to be two channels measuring the same temperature. 13.A method according to any preceding claim in which the two channels arecaused to see substantially the same temperature by: (i) defocusing theradiometer; or (ii) switching the radiation incident upon the detectorsfrom being scene radiation to reference radiation.
 14. A methodaccording to claim 13 in which reference radiation is provided a thermalsource, the temperature of the thermal source is measured, and thetemperature of thermal source is used to determine the gain and offsetfor the channels.
 15. A method of compensating a multi-channelradiometer for variations in the output voltage of a detector withtemperature of the radiometer, comprising modifying the gain and offsetvoltage used in the evaluation of an observed scene temperature bycalibrating one channel's gain and offset relative to another channel'sgain and offset.
 16. A method according to claim 15 further comprisingensuring that the two channels observe the same temperature during acalibration operation.
 17. A multi-channel imaging radiometer having afirst channel having a first channel detector and being adapted to beconnected to a first channel observed scene radiation capturer, and asecond channel having a second channel detector and being adapted to beconnected to a second channel observed scene radiation capturer, and asignal processor, in which observed scene radiation signals are adaptedto be fed to the first and second detectors respectively by their firstand second channels, and the detectors are adapted in use to providefirst and second detector outputs to the signal processor, the signalprocessor being adapted in use to provide output signals representativeof the temperatures observed in the observed scene by the first andsecond channels; and in which the signal processor is adapted in use tooperate on the received first and second detector outputs using valuesrepresentative of, influenced by, or associated with the gain andvoltage offset for the first channel and for the second channel tocalibrate the gain and voltage offset for the second channel relative tothose of the first channel.
 18. A radiometer according to claim 17 inwhich the signal processor is adapted to use the equation: V_(detector)=gain×observed scene temperature+V _(offset), or anequivalent, for each channel to determine the modified gain and offset.19. A radiometer according to claim 17 or claim 18 which is adapted toscan an observed scene in such a way as to overlap or cross scan paths,so that the same point in the observed scene is viewed by more than onechannel.
 20. A radiometer according to claim 19 which is adapted toperform a conical scan pattern.
 21. A radiometer according to any one ofclaims 17 to 20 which is provided with an image-forming focuser assemblyand a defocuser adapted to defocus the detected image.
 22. A radiometeraccording to any of claims 17 to 21 which has an observed radiationdiverter adapted in use to divert the radiation which encounters thedetectors from being radiation which has originated from the scene tobeing radiation that is not from the observed scene.
 23. A method ofimproving the accuracy of an image or of output signals image producedby a radiometer comprising periodically performing an absolutecalibration of the gain and offset voltage applicable to at least oneradiation detection channel against a source of known temperature, theabsolute calibration being performed with the radiometer in situo,without returning the device to the factory or the laboratory forcalibration.
 24. A method according to claim 23 in which the source is athermal source.
 25. A method according to claim 23 in which the sourcecomprises the sky and the method comprises ensuring that radiation fromthe sky is received by the detection channel.
 26. A method according toany one of claims 23 to 25 comprising observing two different thermalsource temperatures.
 27. A radiometer having at least one detectionchannel and a detector adapted to receive radiation acquired in use bythe channel, a control processor, and an absolute reference temperatureprovider, the arrangement being such that the radiometer can be put, inuse, in a calibration mode in which it resets the gain and/or offsetassociated with the, at least one, or each, channel by using signalsderived from the reference temperature provider.
 28. A radiometeraccording to claim 27 which has a selector adapted in use to selectwhether radiation reaches the detector from the observed scene or fromthe reference temperature provider.
 29. A radiometer according to claim27 or claim 28 which is provided with a thermal source of knowntemperature.
 30. A radiometer according to any one of claims 27 to 29 inwhich there is a defocuser.
 31. A method of improving the performance ofa radiometer having at least one detection channel comprising providingcompensation for variations in temperature of the radiometer, or atleast for variation in temperature of at least one component of theradiometer.
 32. A method according to claim 31 comprising providing anequivalence function which in the evaluation of an observed scenetemperature observed by the channel compensates for the temperature ofthe radiometer, or said at least one component.
 33. A method accordingto claim 31 or claim 32 in which the component is (i) an amplifier whichamplifies signals detected by a radiation-feed, or (ii) a detector whichdetects signals obtained from a radiation-feed, or (iii) aradiation-feed.
 34. A method according to any one of claims 31 to 33having a concordance between detector output and evaluated scenetemperature dependent upon the temperature of the radiometer or saidcomponent.
 35. A method according to claim. 34 as it depends from claim33 comprising having a concordance between the input and output of anyof (i), (ii), or (iii) dependent upon the temperature of the component(i), (ii), or (iii).
 36. A method according to any one of claims 33 to35 comprising taking the output of component (ii) and applying amodification which uses one of more of: (a) the input-outputcharacteristics of component (ii) with temperature; and/or (b) theinput-output characteristics of component (i) with temperature; and/or(c) the input-output characteristics of component (iii) withtemperature.
 37. A radiometer having at least one detection channelhaving a gain and offset that are dependent upon the temperature of theradiometer and/or upon the temperature of at least one component of theradiometer, a signal processor, and a temperature sensor adapted todetect the temperature of the radiometer or of said component; thechannel being adapted in use to provide signals indicative of thetemperature in an observed scene to the signal processor, and the signalprocessor being adapted in use to produce evaluated scene temperaturesignals which are dependent upon both the channel signal and upon thetemperature sensor signal.
 38. A radiometer according to claim 37 inwhich (i) the channel has an amplifier and a temperature sensor adaptedto sense the temperature of the amplifier; and/or (ii) the temperaturesensor is adapted to detect the temperature of the detector. (iii)channel has a radiation-acquiring antenna and a temperature sensor isprovided adapted to detect the temperature of that.
 39. A radiometeraccording to claim 38 in which the signal processor has a concordancelinking output detector voltage with evaluated scene temperature signalfor different temperatures of the radiometer or of the or each componentwhich has its temperature input to the signal processor.
 40. A method ofcalibrating a radiometer having a detector channel having a detectorfeed and a detector, and a scanner which scans an observed scene bydirecting different parts of the scene onto the detector feed of thedetector channel at different times, with each notional pixel in anobserved scene observed by the channel having an associated scan angle,the method comprising modifying the observed scene temperature detectedby the detector by a calibration or compensation function which isdependant upon the scan angle of the notional pixel being observed. 41.A method according to claim 40 which comprises having a plurality ofdetector channels and modifying the respective detected scenetemperatures by respective scan angle dependant calibration ormodification functions.
 42. A method according to claim 41 comprisinghaving different detector channels which have a different scan angledependant modification or calibration functions.
 43. A method accordingto any one of claims 40 to 42 in which the calibration or modificationfunction comprises a modification to the gain and/or offset voltage of achannel for the particular pixel concerned.
 44. A scanning imagingradiometer having a scanner which is adapted to scan notional pixels inan observed scene onto a detector element of a detector channel, and acontrol processor that is adapted to receive signals from the detectorelement and is adapted to evaluate the scene temperature of the notionalpixel from the output of the detector element and from a concordance oralgorithm which compensates for the scan angle at which the pixel wasevaluated.
 45. A radiometer according to claim 44 in which theconcordance or algorithm also compensate for the temperature oftemperature sensitive components of the radiometer.
 46. A signalprocessor, or software, which when operatively installed in a radiometerprovides a radiometer or a method in accordance with any precedingclaim.